Russian President Vladimir Putin is calling up thousands of extra troops to fight in Ukraine after suffering setbacks on the battlefield.
Mr Putin said the partial mobilisation was necessary to ensure Russian territorial integrity.
Ukrainian president Volodymyr Zelensky said it showed Mr Putin wanted to drown Ukraine in blood - including that of his own soldiers.
The announcement applies to 300,000 military reservists.
They make up a fraction of the about 25 million Russian reservists - people who have done their military service which is compulsory in Russia.
The mobilisation is the first since World War II and comes after Ukraine made gains in a rapid counter-offensive this month, putting the Kremlin on the back foot.
Ukrainian forces have recaptured key towns and villages in the northern Kharkiv region and have made a slower, but still significant progress in the southern Kherson region. Russia still holds about a fifth of the country.
The decree is short on detail. It says nothing about a cap on numbers or about any exceptions, such as not recruiting students or conscripts.
Instead, these details are left to regional heads to decide how to meet quotas. In theory, the net could be cast far wider than the Kremlin has specified.
However Russian officials said it would announce "very soon" those who would be exempt from its partial mobilisation.
The partial call-up stops short of full conscription, a move that would have risked turning a public that has so far largely been in favour of the conflict against it.
In a televised address, Mr Putin also issued a thinly veiled he could use nuclear weapons.
He said the West was engaging in "nuclear blackmail" and that Moscow had "lots of weapons to reply".
"When the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people. It's not a bluff," he said.
Mr Putin's announcement of a partial mobilisation drew immediate condemnation from Ukraine's allies.
Dutch Prime Minister Mark Rutte called the mobilisation "a sign of panic" while German Chancellor Olaf Scholz called it "an act of desperation".
Mr Putin's address has raised fears that some men of fighting age would not be allowed to leave Russia, even though Russia's defence minister, Sergey Shoigu, said the call-up would be limited to those with combat experience.
He declined to comment on whether borders would be closed to those the call-up would be applicable to.
Flights out of Russia sold out fast following the announcement.